This invention relates to monitoring the hydraulic fracturing of a subsurface formation and more particularly to the use of an aluminum activation log for measuring the extent of such hydraulic fracturing operation.
In the completion of wells drilled into the earth, a string of casing is normally run into the well and a cement slurry is flowed into the annulus between the casing string and the wall of the well. The cement slurry is allowed to set and form a cement sheath which bonds the string of casing to the wall of the well. Perforations are provided through the casing and cement sheath adjacent the subsurface formation. Fluids, such as oil or gas, are produced through these perforations into the well.
It is oftentimes desirable to treat subterranean formations in order to increase the permeability thereof. For example, in the oil industry it is conventional to hydraulically fracture a well in order to produce one or more fractures in the surrounding formation and thus facilitate the flow of oil and/or gas into the well or the injection of fluids such as gas or water from the well into the formation. Such hydraulic fracturing is accomplished by disposing a suitable fracturing fluid within the well opposite the formation to be treated. Thereafter, sufficient pressure is applied to the fracturing fluid in order to cause the formation to break down with the attendant formation of one or more fractures therein. Simultaneously with or subsequent to the formation of the fracture a suitable carrier fluid having suspended therein a propping agent or proppant such as sand or other particulate material is introduced into the fracture. The proppant is deposited in the fracture and functions to hold the fracture open after the fluid pressure is released. Typically, the fluid containing the proppant is of a relatively high viscosity in order to reduce the tendency of the propping agent to settle out of the fluid as it is injected down the well and into the fracture.
Several such hydraulic fracturing methods are disclosed in U.S. Pat. Nos. 3,965,982; 4,067,389; 4,378,845; 4,515,214; 4,549,608 and 4,685,519, for example.
The extent of such formation hydraulic fracturing and the location of proppant materials has been ascertained by the use of radioactive tracers as described in U.S. Pat. No. 3,987,850 to J. L. Fitch. Such radioactive tracers have been plated or coated on propping agents and injected along with the fracturing fluid. These coatings have contained radioactive isotopes such as Au-198, Ag-110, Ir-192, and Sc-46 with half-lives from about 2 to 250 days.
In U.S. Pat. No. 3,796,883 the effectiveness and competency of a well gravelpack and changes therein are determined by monitoring the location of radioactive pellets within the gravel pack.
The use of such radioactive tracers or coatings has, however, presented monitoring, logistical and environmental problems. The short half-lives of such tracers prevent the monitoring of the movement of such tracers in the formation fractures other than over a short time interval. Transportation and use of radioisotopes is expensive and government regulations or restrictions must be accommodated. Disposal of excess radioactive proppants can be a problem, especially in offshore operations.
It is therefore a specific object of the present invention to provide a method for hydraulic fracturing in which the extent of such fracturing is measured without the use of radioactive isotope tracers.